Method and system for optically detecting an electric current by means of light signals having different wavelengths

ABSTRACT

The method and system serve to optically detecting an electric current with an extended measurement range. To that end, two light signals having different wavelengths pass through a Faraday element, the polarization of the two light signals being rotated in each case by at least 0.0014°/A as a function of the electric current. By taking account of both wavelength-dependent polarization rotations, at least one of which lies beyond an unambiguity range, the measurement range is extended distinctly beyond the unambiguity range of each of the two light signals.

This application claims priority to International Application No.PCT/DE99/03976 which was published in the German language on Jun. 292000.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a system and method for optically detecting foroptically detecting an electric current, and in particular, to opticallydetecting an electric current by using light signals having differentwavelengths.

BACKGROUND OF THE INVENTION

WO 98/38517 A1 describes an arrangement for measuring current, in whichtwo light signals each having a different wavelength are generated andare fed via a coupler into an optical waveguide. The optical waveguideserves as a common feed to a Faraday element. Before entering theFaraday element, both light signals are linearly polarized. In thisconnection, linear polarization is to be regarded as a particularlyfavorable special form of elliptical polarization. Generally, however,any other elliptical polarization form is equally suitable as long as ithas a marked direction. The first light signal has a wavelength ofbetween 630 and 850 nm and the second light signal has a wavelength ofbetween 1300 and 1550 nm. On account of the wavelength-dependentVerdet's constant, the polarizations of the two light signals arcinfluenced to different extents in the Faraday element. The changes inpolarization caused by the current are evaluated in a single-channel ortwo-channel manner. In an evaluation unit; the two light signals areseparated from one another in accordance with their wavelength by meansof optical filter elements or by means of the sensitivity range of thephotodiodes used and are converted into electrical signals for furtherprocessing. In this case, the wavelength difference between the twolight signals has the effect that the electrical signal derived from thefirst light signal is an unambiguous function of the current to bemeasured and the electrical signal derived from the other light signalis a non-unambiguous, periodic function of the current to be measured.By the same token, however, the second derived electrical signal has ahigher measurement resolution. From these two electrical signals, ameasurement quantity for the electric current is derived with a largemeasurement range and also with a high measurement resolution. Themethod used in this case is disclosed in DE 195 44 778 A1.

DE 195 44 778 A1 describes a magneto-optical current converter havingtwo Faraday elements for deriving two different measurement signals. Thefirst Faraday element yields a first measurement signal which, in apredetermined measurement range, is an unambiguous function of theelectric current to be measured (=operation in the unambiguity range).By contrast, the second Faraday element is configured in such a way thata second measurement signal that it generates is a non-unambiguous,essentially periodic function of the electric current (=operation in theambiguity range). A third measurement signal for the electric current isbuilt up from the two measurement signals. The third measurement signalis in the predetermined measurement range. Both signals are anunambiguous function of the measurement quantity and have the same highmeasurement resolution as the second measurement signal. However, themethod described requires a relatively high outlay since two separateFaraday elements. A high computation complexity is also required in theformation of the third measurement signal in an evaluation unit.

Moreover, EP 0 210 716 A1 describes a magneto-optical current sensorwhich is operated with two light signals having different wavelengths.In this case the two wavelengths do not serve for extending themeasurement range, but for drift compensation. The Faraday element isagain operated only in the linear, i.e. unambiguous, range of thecharacteristic curve.

DE 31 41 325 A1 discloses a heterodyne method for optical currentmeasurement, in which two light signals having the same wavelength butdifferent intensity modulation are generated. The frequency differenceof the intensity modulation between the two light signals is between 1kHz and 1 MHz. From these two light signals there is generated a furtherlight signal with a linear polarization vector which rotates with thedifferential frequency of the two intensity modulations about thedirection of propagation of the further light signal. The light signalwith rotating linear polarization vector is then both fed into a Faradayelement and transmitted as reference signal directly to an evaluationunit. The electric current is then determined by phase comparisonbetween the reference signal and the light signal emerging from theFaraday element. This method also makes it possible to extend themeasurement range for the optical current measurement beyond theunambiguity range. However, this method is relatively complex owing tothe reference signal and the phase-comparison measuring devices requiredin the evaluation unit.

WO 98/05975 A1 describes a method and an arrangement for optical currentdetection, in which two optical measurement signals are generated as afunction of the electric current to be measured. The dependence of thetwo optical measurement signals on the electric current is in each caseperiodic, the two periods differing from one another at most by thefactor 2. From the two optical measurement signals there are derivedvalue pairs to which it is then possible to assign in each case apresent value of the electric current to be measured. The proceduredescribed by this method likewise makes it possible to extend themeasurement range in the case of optical current detection. However, WO98/05975 A1 contains no embodiments which are distinguished, for exampleby particular efficiency.

SUMMARY OF THE INVENTION

In one embodiment of the invention, there is an electric currentgenerates at least one first elliptically polarized light signal havinga first polarization and a first wavelength and a second ellipticallypolarized light signal having a second polarization and a secondwavelength, which is different from the first wavelength. The electriccurrent feeds the first and the second light signal into a Faradayelement, changes the first and the second polarization as a function ofthe electric current upon passage through the Faraday element, andderives a measurement signal for the electric current from the changesin polarization of the two light signals. The first and the secondpolarization are rotated by at least 0.0014° per ampere of the electriccurrent, and at least one of the two polarizations is rotated by morethan 45° under the influence of a maximum electric current.

In one aspect of the invention, the first and the second polarizationare rotated in the Faraday element by a first and a second angle ofrotation. The first and the second angle of rotation differ at most by afactor 2 given at a predetermined electric current.

In another aspect of the invention, there is a wavelength differencebetween the first and the second wavelength of at most 15% of an averagevalue of the first and second wavelengths. In another aspect of theinvention, the first and the second light signals pass through theFaraday element simultaneously.

In yet another aspect of the invention, the first and the second lightsignals pass through the Faraday element cyclically alternately.

In a further aspect of the invention, the first and the second lightsignal are generated from an optical swept-frequency signal having avarying wavelength. The varying wavelength is tuned between the firstwavelength and the second wavelength.

In a further aspect of the invention, the varying wavelength of theoptical swept-frequency signal is tuned periodically between the firstwavelength and the second wavelength.

In yet another aspect of the invention, the first and the second lightsignal are intensity-modulated during generation with a first and asecond frequency.

In another embodiment of the invention, a transmitting device generatesat least one first elliptically polarized light signal having a firstpolarization and a first wavelength and a second elliptically polarizedlight signal having a second polarization and a second wavelength, whichis different from the first wavelength. There is a Faraday element,which is assigned to the electrical conductor and through which thefirst and the second light signal pass, the Faraday element effecting achange in the first and second polarization as a function of theelectric current to be detected and of a wavelength-dependent effectiveVerdet's constant; and an evaluation unit to derive a measurement signalfor the electric current from the changes in the first and secondpolarization. The effective Verdet's constant of the Faraday element hasa value of at least 0.0014°/A for both wavelengths, and the Faradayelement rotates at least one of the two polarizations by more than 45°given the electric current.

In one aspect of the invention, values of the effective Verdet'sconstant for the first and the second wavelength differ at most by thefactor 2.

In another aspect of the invention, the transmitting device isconfigured to generate the first and the second light signal with awavelength difference of at most 15% of an average value of the firstand second wavelength.

In another aspect of the invention, the transmitting device isconfigured to simultaneously feed the first and the second light signalinto the Faraday element. The transmitting device is configured tocyclically alternately feed the first and the second light signal intothe Faraday element. The transmitting device is further configured tocyclically alternately feed the first and the second light signal andcomprise a tunable light source to generate an optical swept-frequencysignal having a varying wavelength, the varying wavelength varyingbetween the first and the second wavelength. The tunable light source isconfigured to generate an optical swept-frequency signal having aperiodically varying wavelength. The transmitting device comprises amodulation device to intensity modulate the first light signal with afirst frequency and the second light signal with a second frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments will now be explained in more detailwith reference to the drawing for illustration, the drawing is not toscale, and certain features are represented diagrammatically. In detail:

FIG. 1 shows an arrangement for optical current measurement with twolight signals having different wavelengths which are fed simultaneouslyinto a Faraday element.

FIG. 2 shows an arrangement for optical current measurement with twolight signals having different wavelengths which are fed alternatelyinto a Faraday element.

FIG. 3 shows an arrangement for current measurement with a light signalhaving a varying wavelength which is fed into a Faraday element.

FIG. 4 shows an arrangement for optical current measurement with twointensity-modulated light signals having different wavelengths which arefed simultaneously into a Faraday element.

Mutually corresponding parts are provided with the same referencesymbols in FIGS. 1 to 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention specifies a method and an arrangement for opticallydetecting an electric current with the highest possible measurementresolution and a measurement range which goes beyond the unambiguityrange determined by the Faraday element used and the light wavelengthused. Moreover, the intention here is for the optical detection of theelectric current to be effected as efficiently as possible.

In the case of the method according to the invention, the first and thesecond polarization are rotated by at least 0.0014° per ampere of theelectric current to be detected. Moreover, at least one of the twopolarizations is rotated by more than 45° under the influence of amaximum electric current to be detected.

For each Faraday element, there is a preferred sensitivity range inwhich, with the aid of two light signals having different wavelengths,the measurement range can be extended in a particularly simple andefficient manner. In this case, the specific embodiment of the Faradayelement is not important as long as the Faraday element is transparentto both wavelengths. Specifically, all known embodiments can be designedwith a sensitivity range that is particularly favorable with regard tomaximum extension of the measurement range.

If the Faraday element is chosen such that the polarization of the twolight signals having different wavelengths is rotated by more than0.0014°/A under the influence of the electric cart to be measured, thenan unambiguity range of ≦approximately 20 kA (=root-mean-square value)results for a periodic electric current to be detected. The unambiguityrange is determined by the angle-of-rotation range of in whichpolarization rotations caused by the Faraday element are unambiguous.The maximum value—to be detected unambiguously—of the electric currentis then calculated, with the lower limit for the sensitivity of theFaraday element, as:$\frac{45{^\circ}}{0.0014{^\circ}\text{/}A} \leq {\text{approx.}\quad 32\quad {{kA}\left( {= \text{maximum value)}} \right.}}$

In order then to achieve a resolution of, for example, less than 10 A,the unambiguity range is chosen to be as small as possible. This isachieved by choosing the highest possible sensitivity (distinctly abovethe lower limit of 0.0014°/A. A very high measurement resolution of e.g.<1 A can then be obtained in the unambiguity range.

Despite the high sensitivity, it is then possible to achieve a maximumdetectable electric current of up to a few 10 kA or even up to 100-200kA, since the two light signals having different wavelengths can be usedfor extending the measurement range. This is because, given a highelectric current, the Faraday element effects a rotation of theassociated polarization by more than 45° at least in the case of one ofthe two light signals. As a result, the change in polarization of atleast this light signal exceeds the limit of the unambiguity range,resulting in ambiguities and thus affording the possibility for thedesired extension of the measurement range.

From the changes in the first polarization and in the secondpolarization which are caused by the current to be measured, it ispossible to derive a value pair which, in the extended measurement range(≦200 kA) specified above, can be allocated an unambiguous value of theelectric current. In this case, the assignment can be effected eithervia a previously stored look-up table, in which the value pairs arestored with the associated electric current intensity, or else via amathematical formula. In addition, a value pair can also be assigned tothe causal electric current via a mathematical search algorithm withstored characteristic curve functions or with a stored model of themagneto-optical transducer.

The large measurement range and the high resolution in the case of smallcurrents, i.e. in the unambiguity range, correspond precisely to therequirements in this regard in the area of public electrical powersupply, so that the method is particularly suitable for such a use.

In an advantageous embodiment, a first and a second angle of rotation bywhich the first and, respectively, the second polarization are rotatedin the Faraday element differ from one another at most by a factor of 2.This is because, for a measurement range that is as large as possible,it is advantageous if characteristic curves which respectively describethe current dependence for the first and the second wavelength differ aslittle as possible. In particular, it is advantageous if theperiodicities of the characteristic curves, which are sinusoidal to afirst approximation disregarding interference-quantity influences, areas close together as possible. The above-mentioned condition results inthe periods of the two sinusoidal characteristic curves. differing atmost by a factor of 2.

Furthermore, it is preferable if the first and the second wavelengthdiffer from one another by not more than 15% with reference to anaverage value of both wavelengths. Since the sensitivity of the Faradayelement depends on the wavelength, the effect achieved with the rangespecified for the first and second wavelengths is that the resultingmeasurement range is at least one order of magnitude larger than theunambiguity range in the case of separate evaluation of the first or ofthe second light signal.

Another embodiment relates to the feeding of the two light signals intothe Faraday element. The two light signals having different wavelengthspass through the Faraday element together, i.e. simultaneously. Forevaluation, optical separation into the two light signals in accordancewith their respective wavelength is performed, for example. As a result,items of measurement information which are obtained on the basis of thefirst and second wavelengths are present at every point in time. Onaccount of the (light) signal discrimination exclusively in thewavelength domain, this refinement involves a wavelength divisionmultiplex method (Wavelength Domain Multiplex=WDM).

In another embodiment, the first and the second light signal are fedinto the Faraday element periodically alternately. In this embodiment,there is available, at each point in time, the item of measurementinformation obtained via one of the two wavelengths. However, by meansof correspondingly rapid changeover between the two light signals, twosuccessively recorded items of measurement information can be assignedto a single current value to be measured, in particular when achangeover frequency is at least twice as large as a frequency value ofa maximum harmonic of a highest frequency component—to be detected—ofthe electric current I. Optical separation of the two light signals isnot necessary in this embodiment. On account of the signaldiscrimination in the time domain, this embodiment involves a timedomain multiplex method (Time Domain Multiplex=TDM).

In addition, an embodiment in which an optical swept-frequency signalhaving a varying wavelength is sent through the Faraday element is alsoadvantageous. Such a swept-frequency signal can be generated, forexample, by means of a wavelength-tunable light source (laser diode,LED, SLD (superluminescent diode) with tunable transmission filter, andtunable fiber laser (TFL)). In this case, the narrowband varyingwavelength of the swept-frequency signal assumes wavelength valuesbetween the first and second wavelengths. The optical swept-frequencysignal can be imagined as including infinitely many individual signalseach having mutually different individual wavelengths. With thisinterpretation, the first and the second light signal having the firstand, respectively, the second wavelength are then those individualsignals in which the direction of the wavelenth variation is exactlyreversed. Feeding theoretically infinitely many individual wavelengthsinto the Faraday element has the effect that, during evaluation, thereis greater leeway for extending the measurement range, since, inprinciple, an item of measurement information with in each case amutually different dependence on the electric current to be detected ispresent for all of the individual wavelengths.

A refinement in which the wavelength of the optical swept-frequencysignal is varied periodically alternately is preferred.

In a further advantageous refinement, the intensity of the first lightsignal is modulated with a first frequency and the intensity of thesecond light signal is modulated with a second frequency, which differsfrom the first frequency. This intensity modulation is effected duringthe generation of the two light signals. To that end, the amplitude of asupply current which excites a light source such as e.g. a laser diodeor an LED to effect light emission is varied with the correspondingfrequency. The two light signals having mutually different wavelengthsand intensity modulation are then fed into the Faraday element. This canbe effected either simultaneously or cyclically alternately. Forevaluation, the items of measurement information contained in the twolight signals are separated from one another by electrical bandpassfiltering which is carried out after optoelectrical conversion. In thiscase, the center frequencies of the bandpass filters correspond to therespective frequency of the intensity modulation. Optical filtering ofthe two light signals is thereby obviated.

With respect to electrically detecting an electric current in anelectrical conductor, the effective Verdet's constant of the Faradayelement has a value of at least 0.0014°/A for both wavelengths, and theFaraday element rotates at least one of the two polarizations by morethan 45° given a maximum electric current to be detected.

Such an arrangement makes it possible, in a particularly simple and alsoefficient manner, to extend the measurement range in conjunction withunchanged good measurement resolution.

A transmitting device to generate the two light signals having mutuallydifferent wavelengths comprise commercially available light sources suchas, for example, two LEDs, two laser diodes or two SLDs. These lightsources preferably emit in the infrared wavelength range atapproximately 800 nm or at approximately 1300 nm. However, a differentwavelength range is equally well suited. In this case, the possibleemitted wavelengths are subject to a condition insofar as the effectiveVerdet's constant of the Faraday element should have in each case atleast the above-mentioned value of 0.0014°/A at these wavelengths.

In addition, the transmitting device to generate the two ellipticallypolarized light signals also comprise a linear polarizer if the lightemitted by the light source is unpolarized.

The Faraday element can be embodied as a fiber coil, e.g. made of aquartz glass fiber, as a solid glass ring e.g. made of flint glass, oras a magneto-optical bulk single crystal e.g. made of a BSO (bismuthsilicone oxide), a BGO (bismuth germanium oxide) or a YIG (yttrium irongarnet). In this case, the magneto-optical bulk single crystal may alsobe situated in an air gap of a magnetic flux concentrator enclosing theelectrical conductor.

In order to meet the abovementioned condition for the effective Verdet'sconstant, by way of example in the case of a fiber coil made of a quartzglass fiber, the number of turns has to be at least 10 if the twowavelengths used are of the order of magnitude of 800 nm. If wavelengthsof the order of magnitude of 1300 nm are envisaged, then the number ofturns correspondingly has to be at least 25. By contrast, a suitableglass ring in which the two light signals are passed once around theelectrical conductor is composed of flint glass. By way of example,flint glass from Schott with the type designation SF6 or SF57 can beused here. Such a glass ring is intended for operation at wavelengths ofthe order of magnitude of 485 nm. Owing to the, in general, non-existentpossibility for increasing the sensitivity (=effective Verdet'sconstant) over a repeated light circulation, such a glass ring, despitethe higher material-specific sensitivity of flint glass compared withquartz glass, meets the condition for the lower limit of sensitivity(=0.0014°/A) at distinctly shorter wavelengths than is the case with aquartz glass fiber coil. Free design parameters are thus the geometry(=e.g. number of light circulations) of the Faraday element, thematerial-specific properties of the Faraday element and the wavelengthsused.

The evaluation of the changes in the first and in the secondpolarization which are caused by the current to be measured is performedin an evaluation unit. The evaluation unit determines a value pair fromthe changes in polarization and then assigns a corresponding value ofthe electric current to the value pair using a stored look-up table. Inthis case, the evaluation unit may be realized using analog and/ordigital circuitry. In particular, the evaluation unit may comprise adigital calculation unit, for example in the form of a digital signalprocessor.

An existing arrangement for optical current measurement can also beextended in a simple manner in the measurement range, since nointervention is necessary in the actual optical path. Rather, theextension of the measurement range is achieved solely by exchanging theexist transmitting, receiving and evaluating devices.

In a preferred embodiment, the transmitting device also comprises a unitfor simultaneously feeding the two light signals into the Faradayelement. In this case, a coupler may be involved which feeds the lightsignals originating from different light sources into a common opticalwaveguide leading to the Faraday element.

FIG. 1 illustrates an arrangement for optically measuring an electriccurrent I flowing in a current conductor 5. Two light signals having afirst wavelength λ1 and a second wavelength λ2, respectively, aregenerated in a first light source 11 and a second light source 12. Thetwo light signals are fed to a polarizer 21 via a coupler 31, which isdesigned as a 2×1 fiber coupler in the present case. A first lightsignal L1 having a first linear polarization and the first wavelength λ1and also a second light signal L2 having a second linear polarizationand the second wavelength λ2 are then present at the output of thepolarizer. Instead of the linear polarization, elliptical polarizationwith a preferred direction is also possible.

The two linearly polarized light signals L1 and L2 are then fed to aFaraday element 10, which is assigned to the current conductor 5. Theyare fed jointly in this case, for example via an optical waveguide (notspecifically illustrated). In another exemplary embodiment (notillustrated), the linear polarization of the two light signals is notgenerated until directly before entry into the Faraday element 10.

In the Faraday element 10, the linear first and second polarizations ofthe first and, respectively, second light signals L1 and L2,respectively, are rotated about a first and, respectively, a secondangle ρ1 and ρ2 of rotation. In this case, the angles ρ1 and ρ2 ofrotation are each dependent on the magnitude of the electric current Iand on a constant determined by the Faraday element 10, a so-calledVerdet's constant V. The Verdet's constant V is dependent on a materialused in the Faraday element 10, on an ambient temperature and on a lightwavelength λ radiated through the Faraday element 10. In particular, thedependence on the light wavelength λ is of particular importance for theexemplary embodiment—illustrated in FIG. 1—of a magneto-optical currenttransducer with an extended measurement range. The following empiricallydetermined relationship can be specified for the wavelength dependenceof the Verdet's constant V: $\begin{matrix}{{V(\lambda)} = {{\frac{180}{\lambda} \cdot \frac{{n^{2}(\lambda)} - 1}{n(\lambda)}}\left( {A + \frac{B}{\lambda^{2} - \lambda_{c}^{2}}} \right)}} & (1)\end{matrix}$

where n(λ) designates a wavelength-dependent refractive index of thematerial used and A, B and λ_(c) designate material constants which canbe taken for example from a relevant table work.

For a Faraday element 10 designed as a fiber coil with N turns, thefirst angle ρ1 of rotation for the first light signal L1 then turns outto be:

 ρ1=N·V(λ1)·I.  (2)

For the second light signal having the second light wavelength λ2, asecond angle ρ2 of rotation correspondingly turns out as:

ρ2=N·V(λ2)·I.

From the wavelength-dependent Verdet's constant V and the number N ofturns it is possible to form an effective Verdet's constant Veff, sothat the first and the second angle of rotation, ρ1 and ρ2 respectively,depend exclusively on this wavelength-dependent effective Verdet'sconstant Veff and the electric current I to be measured. In acorresponding manner, an effective Verdet's constant Veff can also bedefined for other embodiments of the Faraday element 10, for example aglass ring or a magneto-optical bulk crystal, which constant thenincludes the respective geometry-dependent influencing variables.

The specialist literature also includes a definition—which is differentfrom equations (2) and (3)—of Verdet's constant V as a material-specificwavelength-dependent proportionality constant which specifies therelationship between the angle of rotation and the product of magneticinduction and light path length. However, both definitions of Verdet'sconstant V can be converted into one another in a simple manner.

After passing trough the Faraday element 10, the two light signals L1and L2, whose polarization has been rotated, are evaluated in anevaluation unit 20. In an analyzer 22, which is designed as WollastonPrisma in the present case, the two light signals L1 and L2 are first ofall split into two light signal elements LT1 and LT2 having mutuallyperpendicular planes of polarization. This corresponds to the evaluationwhich is designated as “two-channel” above. If, in another exemplaryembodiment (not illustrated), the analyzer 22 allows the two lightsignals L1 and L2 whose polarization has been rotated to pass withregard to one plane of polarization, then, by contrast, single-channelevaluation is present.

The two light signal elements LT1 and LT2 are split intowavelength-selective components by couplers 32 and 33 and opticaltransmission filters 34, 35, 36 and 37 tuned to the first and secondwavelengths. Optoelectrical transducer units 41, 42, 43 and 44 convertthe components into electrical signals I11, I12, I21 and I22 which eachcomprise an item of information about the first and, respectively, thesecond angle ρ1 and ρ2 of rotation caused by the current I to bemeasured at the respective wavelength λ1 and λ2. The opticaltransmission filters 34, 35, 36 and 37 are embodied as interferencefilters. However, they may likewise be realized as cut-off filters. Theoptoelectrical transducer units 41, 42, 43 and 44 each comprise aphotodiode tuned to the corresponding wavelength λ1 or λ2, and atransimpedance amplifier connected downstream.

In a signal processing unit 101, a measurement signal M for the electriccurrent I is derived from the electrical signals I11, I12, I21 and I22.In order to eliminate interference-quantity influences, intensitynormalization is firstly provided in the signal processing unit 101prior to the actual evaluation. However, this intensity normalization isnot absolutely necessary, it can also be omitted. The subsequent actualdetermination of the measurement signal M is effected according to amethod disclosed in the prior art. In order to carry out thisdetermination, the signal processing unit 101 also contains a digitalcalculation unit in the form of a digital signal processor.

Disregarding the interference-quantity influences, which are not ofinterest in this connection, the electrical signals I11, I12, I21 andI22 have an essentially sinusoidal dependence on twice the first orsecond angle of rotation, ρ1 or ρ2 respectively. An unambiguity rangedetermined by the sine function is thus obtained for the electriccurrent I to be measured. The measurement range is initially limited tothis unambiguity range. However, the arrangement illustrated in FIG. 1serves precisely to extend the measurement range beyond said unambiguityrange in a particularly simple and efficient manner.

For this purpose, the Faraday element 10 is dimensioned precisely suchthat the following holds true for its effective Verdet's constant Veffat both light wavelengths λ1 and λ2:

Veff≧0.0014°/A.  (4)

The dependence of Verdet's constant V on the wavelength λ as specifiedin equation (1) is based, in principle, on a proportionalityrelationship in accordance with: $\begin{matrix}{{V(\lambda)} \propto \frac{1}{\lambda^{2}}} & (5)\end{matrix}$

This becomes apparent if the wavelength dependence of the refractiveindex n(λ) is also taken into consideration in equation (1). For aFaraday element 10 designed as a fiber coil or glass ring, the followingrelationship can be derived using equation (5) for the effectiveVerdet's constant Veff: $\begin{matrix}{{{Veff}(\lambda)} = {V_{0} \cdot N \cdot \left( \frac{\lambda_{0}}{\lambda} \right)^{2}}} & (6)\end{matrix}$

where V₀ designates a known value of Verdet's constant V at a givenwavelength (λ₀). The value pair (V₀, λ₀) is material-specific in thiscase. N generally denotes the number of closed circulations of the twolight signals L1 or L2 around the electrical conductor 5. In the case ofa fiber coil, N then corresponds to the number of turns, as alreadyspecified in connection with equations (2) and (3). In a glass ring, bycontrast, the light is generally passed once around the conductor 5,through which current flows, with the result that N assumes the valueone in this case.

For quartz glass and flint glass as light-guiding media, a Verdet'sconstant V₀ of about 0.00015°/A and of about 0.00075°/A, respectively,are known, for example, at a wavelength λ₀ of 820 nm. If these materialparameters are inserted into equation (6) and if account is also takenof the requirement for minimum sensitivity in accordance with equation(4), then a dimensioning specification is obtained for the Faradayelement 10. Free parameters here are the number of light circulationsand the wavelength λ.

Consequently, for a fiber coil which is made of a quartz glass fiber andis operated at 820 nm, the resulting number of turns is at least 10. Ina corresponding manner, for a fiber coil which is made of a flint glassfiber and is operated at the same wavelength, the resulting number ofturns is at least 2. By contrast, for a flint glass ring (N=1), ahighest possible operating wavelength of 600 nm is obtained as aspecification.

The Faraday element 10 shown in FIG. 1 is embodied as a fiber coil madeof a quartz glass fiber having a number of turns N≧10. The two lightwavelengths λ1 and λ2 are 780 nm and 840 nm, respectively. The first andsecond light sources 11 and 12, respectively, are laser diodes whichemit narrow band CW light signals at the wavelengths specified.

Specifically, for extending the measurement range relative to theunambiguity range by at least one order of magnitude, it is advantageousif the difference between the first and the second light wavelength λ1and λ2, respectively, is as small as possible. In order to obtain ameasurement range that is as large as possible, the first and the secondwavelength λ1 and λ2, respectively, are therefore chosen in such a waythat their wavelength difference is at most 15% of the arithmetic meanof both wavelengths λ1 and λ2.

In an exemplary embodiment (not illustrated) in which the twowavelengths λ1 and λ2 are of the order of magnitude of 1300 nm, thefiber coil used as Faraday element 10 then has at least 25 turns inorder to meet the abovementioned condition for the effective Verdet'sconstant Veff.

In the case of the arrangement for detecting an electric current I asillustrated in FIG. 2, the two light signals L1 and L2 are fed into theFaraday element 10 not simultaneously but cyclically alternately. Tothat end, the light sources 11 and 12 are connected via a transmissionchangeover switch 51 to a current source 61, which supplies the lightsources 11 and 12 and excites them to effect light emission. Thetransmission changeover switch 51 is controlled by a control unit 54 insuch a way that one of the two light sources 11 or 12 is supplied withcurrent and thus emits light. The transmission changeover switch 51 andthe control unit 54 thus enable the two light signals L1 and L2 to befed into the Faraday element 10 cyclically alternately. Moreover, if themethod of operation of the arrangement shown in FIG. 1 is referred to aswavelength domain multiplex, then the method of operation of thearrangement of FIG. 2 correspondingly represents time domain multiplexoperation.

Since the two light signals L1 and L2 having different wavelengths λ1and λ2 do not pass through the Faraday element 10 simultaneously, thereis also no need for optical means for wavelength separation in theevaluation unit 20. The electrical signals I11, I12, I21 and I22, whichcarry the measurement information about the electric current I as afunction of one of the two wavelengths λ1 and λ2, are obtained byreception changeover switches 52 and 53, respectively, being connectedbetween the optoelectrical transducer units 41 and 42 and a signalprocessing unit 102. The reception changeover switches 52 and 53 arechanged over by the control unit 54 with the same timing as thetransmission changeover switch 51.

Synchronization of the transmission changeover switch 51 and of thereception changeover switches 52 and 53 is possible without difficultysince the evaluation unit 20 and also the transmitting device togenerate the two light signals L1 and L2 are usually situated in closelocal proximity, in particular even in one and the same assemblyhousing.

The further processing of the electrical signals I11, I12, I21 and I22is effected analogously to the exemplary embodiment of FIG. 1. Likewise,the parameters for the light wavelengths λ1 and λ2 used and also for theeffective Verdet's constant Veff are chosen analogously to the exemplaryembodiment of FIG. 1.

The arrangement of the exemplary embodiment of FIG. 3 uses, instead oftwo light signals L1 and L2 each having a different first and secondwavelength λ1 and λ2, respectively, an optical swept-frequency signal L3having a varying light wavelength λ3. In this case, the varying lightwavelength λ3 periodically assumes values between the first and thesecond wavelength λ1 and λ2, respectively. The transmitting device usedto generate the optical swept-frequency signal L3 is a light source 13which can be tuned in the emitted wavelength and is modulated with acorresponding wavelength modulation signal λmod.

The light source 13 is designed as a TFL, whose emitted narrowbandwavelength can be varied about a central wavelength of e.g. 810 nm. Inthis case, the first and second wavelengths λ1 and λ2, respectively, areagain chosen in such a way that the wavelength difference is at most 15%of the central wavelength. In the exemplary embodiment of FIG. 3, thefirst wavelength λ1 has a value of 800 nm and the second wavelength λ2has a value of 820 nm. Alternative transmitting device to generate theoptical swept-frequency signal L3 are a broadband-emitting light sourcesuch as e.g. a superluminescent diode (SLD) and a modulable narrowbandtransmission filter connected downstream. In this case, the transmissionfilter can be arranged both on the transmitter and on the receiver.

In the Faraday element 10, the polarization of the opticalswept-frequency signal L3 is rotated by a third angle ρ3 of rotation asa function of the electric current I and of the varying light wavelengthλ3. Owing to the varying light wavelength λ3, this angle ρ3 of rotationis based also on a varying characteristic curve instead of a constantone. In the evaluation unit 20, firstly the optoelectrical transducerunits 41 and 42 convert the two light signal elements LT1 and LT2,respectively, into electrical signals I1 and I2, respectively, fromwhich a normalized signal P is generated in a normalization unit 71. Thenormalized signal P carries the measurement information about theelectric current I still as a function of the varying wavelength λ3.Filtering of the normalized signal P in a first low-pass filter 81yields an average-value signal S, which corresponds to an averagecharacteristic curve of the characteristic-curve variation.

In addition, a variation component corresponding to the swing of thecharacteristic-curve variation is extracted from the normalized signal Pin a high-pass filter 82. This swing of the characteristic-curvevariation about the average characteristic curve is determined by thewavelength modulation signal λmod. In a multiplier 84, therefore, thevariation component is superposed with the wavelength modulation signalλmod and subsequently filtered in a second low-pass filter 83. Aquadrant signal Q is determined by this procedure known from quadraturedemodulation. Just like the average-value signal S, said quadrant signalcarries an item of (ambiguous) measurement information about theelectric current I.

From the average-value signal S and the quadrant signal Q, a signalprocessing unit 103 determines the measurement signal M for the electriccurrent I. This is done using a look-up table stored in the signalprocessing unit 103.

The periodic variation—determined by the wavelength modulation signalλmod—of the wavelength λ3 between the first and second wavelengths issinusoidal. However, its profile may also be a sawtooth waveform oranother periodic signal waveform. In this case, a modulation frequencyFmod of the wavelength modulation signal λmod has a frequency valuewhich is at least twice as large as that of a maximum harmonic of ahighest frequency component—to be detected—of the electric current I.Specifically, as soon as the sinusoidal characteristic curve of theFaraday element 10 is modulated beyond the linear range, harmoniccomponents (=bessel components) arise whose frequency is higher, thehigher the modulation is. The above frequency condition appliesanalogously to a change over frequency of the arrangement shown in FIG.2. In the example of FIG. 3, the modulation frequency Fmod has a valueof 200 kHz, for example.

FIG. 4 illustrates an arrangement for optically detecting the electriccurrent I in which the light sources 11 and 12 are supplied viamodulable current sources 62 and 63. The modulable current sources 62and 63 make available to the light sources 11 and 12, respectively, acurrent signal whose amplitude is modulated in a square-wave, sinusoidalor sawtooth manner with a first frequency F1 and, respectively, with asecond frequency F2. These modulated current signals cause the first andsecond light sources 11 and 12, respectively, to emit optical signalswhose intensity is modulated with the first and, respectively, thesecond frequency F1 and F2, respectively. The modulable current sources62 and 63 act as a modulation device for intensity modulation of the twolight signals L1 and L2.

A frequency difference between the two frequencies F1 and F2advantageously lies between 1 kHz and 1 MHz. This reliably avoidscrosstalk. In the present case, a value of 200 kHz is envisaged for thefirst frequency F1 and a value of 300 kHz is envisaged for the secondfrequency F2. In another exemplary embodiment, the first and secondfrequencies F1 and F2, respectively, have a value of 50 and 60 kHz,respectively. In this case, the two frequencies F1 and F2 are againchosen in such a way that their respective frequency value is at leasttwice as large as that of the maximum harmonic of the highest frequencycomponent—to be detected—of the electric current I.

The first and second light signals L1 and L2 fed into the Faradayelement 10 thus differ not only in their wavelength but also in thefrequency of their intensity modulation. The two light signals L1 and L2are fed in simultaneously, analogously to the exemplary embodiment ofFIG. 1. Thus, wavelength domain multiplex operation is again present,although in this case with additional intensity modulation.

In the evaluation unit 20, the electrical signals I11, I12, I21 and I22including the wavelength-related measurement information about theelectric current I are generated by electrical bandpass filtering. Tothat end, the electrical signals 11 and 12 generated from the two lightsignal elements LT1 and LT2 by the optoelectrical transducer units 41and 42 are fed to first bandpass filters 91 and 93, respectively, with acenter frequency corresponding to the first frequency F1, and to secondbandpass filters 92 and 94, respectively, with a center frequencycorresponding to the second frequency F2. Further processing of theelectrical signals I11, I12, I21 and I22 then follows in a signalprocessing unit 104, said further processing essentially correspondingto that described in connection with the exemplary embodiments of FIGS.1 and 2. Likewise, the first and second wavelengths λ1 and λ2,respectively, used in the exemplary embodiment of FIG. 4 and also theFaraday element 10 are dimensioned analogously to the exemplaryembodiments of the preceding figures.

In addition to the transmissive embodiment—described in FIGS. 1 to4—with light being fed in on one side in each case, reflectiveembodiments of the arrangement or embodiments with light being fed intothe Faraday element 10 in opposite directions are also possible.

While the invention has been explained with respect to the embodimentsdescribed above, it will be apparent to those skilled in the art thatvarious modifications and improvements may be made without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the invention is not to be limited by the specificillustrated embodiments, but only by the scope of the appended claims.

What is claimed is:
 1. A method for optically detecting an electriccurrent, comprising: generating at least one first ellipticallypolarized light signal having a first polarization and a firstwavelength and a second elliptically polarized light signal having asecond polarization and a second wavelength, which is different from thefirst wavelength, are generated; feeding the first and the second lightsignal into a Faraday element; changing the first and the secondpolarization as a function of the electric current upon passage throughthe Faraday element; and deriving a measurement signal for the electriccurrent from the changes in polarization of the two light signals,wherein the first and the second polarization are rotated by at least0.0014° per ampere of the electric current, and at least one of the twopolarizations is rotated by more than 45° under the influence of amaximum electric current.
 2. The method as claimed in claim 1, whereinthe first and the second polarization are rotated in the Faraday elementby a first and a second angle of rotation, respectively, the first andthe second angle of rotation differing at most by a factor 2 given apredetermined electric current.
 3. The method as claimed in claim 1wherein there is a wavelength difference between the first and thesecond wavelength of at most 15% of an average value of the first andsecond wavelengths.
 4. The method as claimed in claim 1, wherein thefirst and the second light signals pass through the Faraday elementsimultaneously.
 5. The method as claimed in one of claims 1 wherein thefirst and the second light signals pass through the Faraday elementcyclically alternately.
 6. The method as claimed in claim 5, wherein thefirst and the second light signal are generated from an opticalswept-frequency signal having a varying wavelength, the varyingwavelength being tuned between the first wavelength and the secondwavelength.
 7. The method as claimed in claim 6, wherein the varyingwavelength of the optical swept-frequency signal is tuned periodicallybetween the first wavelength and the second wavelength.
 8. The method asclaimed in claim 1, wherein the first and the second light signal areintensity-modulated during generation with a first and a secondfrequency, respectively.
 9. A system to optically detect an electriccurrent in an electrical conductor, comprising: a transmitting device togenerate at least one first elliptically polarized light signal having afirst polarization and a first wavelength and a second ellipticallypolarized light signal having a second polarization and a secondwavelength, which is different from the first wavelength; a Faradayelement, which is assigned to the electrical conductor and through whichthe first and the second light signal pass, the Faraday elementeffecting a change in the first and second polarization as a function ofthe electric current to be detected and of a wavelength-dependenteffective Verdet's constant; and an evaluation unit to derive ameasurement signal for the electric current from the changes in thefirst and second polarization, wherein the effective Verdet's constantof the Faraday clement has a value of at least 0.0014°/A for bothwavelengths, and the Faraday element rotates at least one of the twopolarizations by more than 45° given the electric current.
 10. Thesystem as claimed in claim 9, wherein values of the effective Verdet'sconstant for the first and the second wavelength differ at most by thefactor
 2. 11. The system as claimed in claim 9 wherein the transmittingdevice is configured to generate the first and the second light signalwith a wavelength difference of at most 15% of an average value of thefirst and second wavelength.
 12. The system as claimed in claim 9,wherein the transmitting device is configured to simultaneously feed thefirst and the second light signal into the Faraday element.
 13. Thesystem as claimed in claim 9, wherein the transmitting device isconfigured to cyclically alternately feed the first and the second lightsignal into the Faraday element.
 14. The system as claimed in claim 13,wherein the transmitting device is configured to cyclically alternatelyfeed the first and the second light signal and comprise a tunable lightsource to generate an optical swept-frequency signal having a varyingwavelength, the varying wavelength varying between the first and thesecond wavelength.
 15. The system as claimed in claim 14, wherein thetunable light source is configured to generate an opticalswept-frequency signal having a periodically varying wavelength.
 16. Thesystem as claimed in claim 9, wherein the transmitting device comprisesa modulation-device to intensity modulate the first light signal with afirst frequency and the second light signal with a second frequency.